Find a good value for maximum timeout for Shared Runners

With GitLab 10.7, we're releasing the Per runner maximum job timeout feature (gitlab-org/gitlab-ce!17221). We should do some calculations and specify a good value that we could set for:

gitlab-shared-runners-manager-X - these runners should be used only for GitLab CE/EE jobs (from main projects and forks), so we should find out the longest time for a job and add few minutes for a safety margin;

shared-runners-manager-X - these are a general purpose runners available for all users. We should check what are the times of jobs currently running on Shared Runners, what timeouts are set in projects that are using the Shared Runners. Ideally would be to get a histogram of such timings. Having this we could decide what value of maximum timeout is a reasonable compromise.